Some combined cycle power plants utilize a single heat source to run two or more heat engines. For example, some combined cycle power plants utilize a gas turbine as the heat source. The gas turbine includes a compressor coupled to a compressor positioned downstream thereof. Rotating elements of the compressor and the turbine are typically mounted on a common rotatable shaft. A combustion chamber is positioned between the compressor and the turbine. Fuel, for example natural gas, is supplied to the combustor. In the combustor, the gas is mixed with air and ignited thereby releasing energy in the form of hot and high velocity combustion gases that are admitted to and turn the rotating elements of the turbine. The turbine can be coupled to an electric generator for production of electricity. The combustion gases exiting the turbine are used in a heat recovery steam generator (HRSG) to provide useful steam for spinning another turbine system, for example a steam turbine. The steam turbine system can be coupled in tandem to the shaft of the turbine in the gas turbine system or can be a standalone unit.
The steam turbine system may include three sections, including a high pressure section, an intermediate pressure section and a low pressure section. Supercritical steam (i.e., at or above the critical point) is admitted to the high pressure section. Since at supercritical pressures and temperatures water and steam cannot be distinguished, the systems and components for collecting and transferring the supercritical steam are simplified, compared to those for handling steam below the critical point. Steam pressure requirements for intermediate pressure section and the low pressure section are successively lower than that of the high pressure section. Each of the three sections has an inlet for admission of the steam to the respective section and an outlet for discharge of steam at a lower pressure and temperature than that at the respective inlet. The pressure and temperature reduction of the steam is effected by the extraction of energy from the steam used to spin the turbine blades in that stage. The steam discharged from the outlet of the high pressure section has useful energy that can be further heated in the HRSG and subsequently supplied to the inlet of the intermediate pressure stage. Low energy steam discharged from the low pressure section is sent to a condenser where the low energy steam is cooled and condensed into condensate that is subsequently supplied to the HRSG for production of steam.